I built a couple 6'x6'x6'.Get a hunting seat you want to use,sit in it,near a wall and you can draw your bow with elbow close to the wall and see how much room you need.keep in mind,you want to sit near the rear and when you extend your bow arm to begin the draw,you don't want your broadhead peaking out the window.
I've shot 60" bows out of them,held vertically,sitting and kneeling.You could go a little longer and if you cant the bow,longer yet.I couldn't use much smaller than a 6' shack.
I'm sure this design can be improved upon but I have these plans so I'll throw them out there.It might give you a starting point.